Nitrogen oxide (NOx) such as nitric oxide (NO) or nitrogen dioxide (NO2) can be formed during coal combustion. Nitrogen oxides can have severe health effects. In addition, nitrogen oxides are identified as greenhouse gases. Atmospheric NOx forms nitric acid which contributes to acid rain. Hence, environmental regulations can call for the reduction of nitrogen oxide formation in combustion reactions in furnaces and boilers and may include even the application of economic fines for emitters.
Nitrogen oxide (briefly NOx or NOx) is, inter alia, formed in so-called hot spots of flames where the temperature is essentially higher than desired. Optical techniques are applied in order to monitor the combustion process given the fact that the reaction leading to nitrogen oxide formation correlates with high temperature spots in the flame. A direct measurement of the hot spots can thus be utilized as a control parameter to optimize the combustion and thereby reduce the formation of NOx.
Further, the formation of carbon monoxide (CO) in the combustion process is an issue that needs attention as well. Both the formation of NOx and CO depends on several variables, in particular on the temperature, the ratio between fuel and air present in the combustion process and the particle size of the fuel.
In order to determine and control these variables, optical monitoring is used in combustion processes. In the combustion of clear burning fuels such as gas and liquid hydrocarbons, optical standard methods such as spectroscopy allow satisfying results in the determination and localization of hot spots. However, the optical monitoring of coal combustion is not comparably straightforward due to the longer volatilisation time of the solid coal. That is, when burning coal, there are always small particles present in the flame complicating spectroscopy.
A further task in coal burning is to identify particle sizes, because particles deviating from the optimum size will further result in non-optimal combustion processes that can result in hot spots or the like. The variability of the particle sizes has also an impact on the combustion control. Smaller particles burn quicker, larger particles need more time to volatize.
Known processes measure optically the emission from radical species present in the flame such as OH−, CH− and C2−3. In combustion of clear burning gases and liquid fuels, monitoring of emission from these radicals in the flame can give an accurate reading of flame temperature and reaction stoichiometry. For coal applications, however, due to the presence of solid particles of varying size, the direct detection of radical species is not comparably feasible. The coal particles scatter the light emanating from the flame thus making the direct measurement of the flame temperature from radicals difficult or even impossible. In addition, stoichiometry involves optical elements such as filters, lenses, mirrors or the like that are expensive and can call for a high degree of maintenance. For instance, in a coal burning power plant which has a large number of burners, providing, setting up and maintaining these optical elements for each burner implies a large investment.
Therefore, there exists a need for an improved optical monitoring method and apparatus capable of determining the temperature, in particular hot spots as well as the particle size of the fuel. In particular, a need exists for an easy to install and comparably cheap optical monitoring method and device so that a multitude of the devices can be installed at several burners.